1. Field of the Invention
This invention relates generally to apparatus for use in determining the front and back contours of the cornea of a human eye and thus facilitating the diagnosis and evaluation of corneal anomalies, design and fitting of contact lens, and the performance of surgical procedures. The present invention also relates to the field of measurement of the refractive characteristics of an optical system, and more particularly, to automatic measurement of the refractive characteristics of the human or other animal eye and to corrections to the vision thereof.
2. Background of Related Art
Corneal Front Surface Measurements
The cornea, being the front surface of the eye, provides the majority of the refracting power (about 2/3) of the eye and is important to quality of vision. Recently, a number of corneal surgical techniques have been developed for correcting visual deficiencies, such as near-sightedness, far-sightedness and astigmatism. In order to assist with such surgical techniques, a number of devices have been proposed or developed to evaluate the topography, i.e., the shape or curvature, of the cornea. In addition, such corneal topography techniques are useful for fitting contact lenses and for the diagnosis and management of corneal pathologic conditions, such as keratoconus and other ectasias. For example, prior to performing a corneal surgical technique to correct a refractive error, the patient is preferably screened using a corneal topography device to rule out the possibility of subclinical keratoconus.
Corneal topography is typically measured using a series of concentric lighted rings, known as a keratoscope pattern, shown in FIG. 12. In a typical embodiment, the keratoscope pattern (reflected image of rings on CCD) is created by a keratoscope target, consisting of illuminated concentric rings which emit light rays which are projected onto the cornea of the patient's eye. Light rays are reflected off the patient's cornea, and a portion of the light is captured by a camera lens and focused onto a CCD. A computer is utilized to analyze the captured image to identify any distortions in the captured image and thus calculate any deformations in the patient's cornea.
While conventional corneal topography devices have achieved significant success, such devices suffer from a number of limitations, which, if overcome, could significantly enhance their accuracy and utility. For example, commercially available topography devices, such as the design illustrated in FIG. 12, typically measure the topography of only a portion of the cornea. In the design shown in FIG. 12, the light beam is emitted from a large, backlit keratoscope target and is then reflected off the cornea. Thereafter, a portion of the light reflects off the cornea and is focused by the camera lens at the center of the keratoscope target onto the CCD. Using this same technique to attempt to image the peripheral portion of the cornea would require a very large extended keratoscope target as shown in the "imaginary extension" of FIG. 12. This imaginary extension could not be realized in a real system due to size and interference with the subject's head. Therefore, such prior art devices are unable to measure the peripheral cornea.
To overcome this problem, other corneal topography devices have attempted to capture the light rays reflected from the peripheral portions of the cornea by designing a very small keratoscope target in the shape of a cylinder or cone, as shown in FIG. 13, encompassing the peripheral cornea. In this manner, light rays emitted by the cylindrical or conical keratoscope target will form a pattern of illuminated rings which will be reflected off the cornea. The reflected light rays, including light rays reflected off the peripheral portions of the cornea, will be captured by the lens and imaged onto the CCD. For this strategy, however, the cylindrical or conical keratoscope target must be positioned very close to the eye, and thereby tends to impinge on the patient's brow and nose. In addition to being potentially uncomfortable and potentially contributing to the spread of disease, the close approach of the keratoscope target makes the design very error-prone, as a slight error in alignment or focusing causes a large percentage change in the position of the keratoscope rings relative to the eye and, hence, a large error in the measurement of the cornea.
In addition, current systems tend to provide poor pupil detection and do not accurately measure non-rotationally symmetric corneas, such as those with astigmatism. The location of the pupil is particularly important in planning surgical procedures for correcting visual deficiencies. In current systems, pupils are typically detected by deciphering the border of the pupil from the image of the keratoscope rings. This is particularly difficult with conventional designs, however, as the intensity transition from the black pupil to a dark iris is minimal compared to the intensity transition from a bright keratoscope ring image to a dark interring spacing. As a result, the pupil detection algorithms in current systems often fail or provide poor results.
In a recent corneal topography advancement, Malone (U.S. Pat. No. 5,873,832) describes a technique which utilizes a virtual image of a keratoscope pattern. The topography system reflects a structured light pattern off the cornea where light rays travel perpendicular to the cornea. In this manner, more of the peripheral cornea is imaged. The geometry of these reflected rays is similar to that of the innermost rays of the traditional corneal topography system. It is well known that the innermost data of traditional corneal topography systems have relatively low accuracy, so it is likely that this new technique will have lower accuracy than that currently provided by the commercially available corneal topography systems.
In the present invention, we overcome these problems of cornea measurement coverage and accuracy using a novel skew-view corneal analysis technique as explained below. We make use of three cameras as was detailed by Sarver (U.S. Pat. No. 5,847,804). While Sarver specifically used a front-view camera and two orthogonal side view cameras (only one of which was used during an exam), the present invention uses a front-view camera and a left- and right-camera oriented at 45 degrees to the optical axis of the front-view camera and all three cameras are used for each exam.
Cornea Back Surface Measurements and Cornea Thickness
Corneal thickness is commonly measured using an ultrasound technique. The hand held A-scan ultrasound probe produces a single-point measurement of the thickness of the cornea. This single point is, in reality, the average thickness of an area of several square millimeters in extent. Because the location of the measurement is dependent upon the operator's positioning, the location of the measurement is not exactly repeatable, hence the data is variable as well.
Another method is the scanning slit technique reported in Snook (U.S. Pat. No. 5,512,966), Knopp (U.S. Pat. No. 5,870,167), and Lempert (U.S. Pat. No. 5,404,884). In these techniques a slit of light is passed through the cornea and the interface of the slit with the front and back surfaces is evaluated from a digitized image. Using this information and an estimate of the index of refraction of the cornea, the thickness of the cornea can be estimated. By scanning the slit over several portions of the cornea, the thickness of a significant portion of the cornea can be obtained. Since the diffuse interaction of the light slit and the cornea can be ill-defined, the image processing will not be exact and so the measurements will contain some amount of error. These techniques also suffer from the characteristic that a large number of images must be obtained and processed to estimate a large portion of the cornea. The result is a large amount of data to process and store as well as the complexity of registration of the images due to movement of the eye during the acquisition period. Even though advanced data compression techniques exist such as that developed by Sarver (U.S. Pat. No. 5,418,714), the images still must be decompressed prior to processing.
In the present invention we take a completely novel approach which eliminates these shortcomings. Using the same three cameras as used in the cornea front surface calculation, we image the pupil contour. A light pattern in the shape of a cross, similar to two simultaneously projected orthogonal slits, is projected onto the cornea. As illustrated in FIG. 14, this pattern is viewed by the front-view camera (C3) to image the horizontal portion of the cross and the left- and right-view cameras (C1 and C2) to image the vertical portion of the cross. This provides a starting point for the corneal thickness and corneal back surface measurements. Then, knowing the front surface, and the starting point provided by the horizontal and vertical thickness data, we find the back surface such that corresponding rays from the three camera views would trace through the cornea and intersect at the pupil contour. Details of this new process are presented below.
Wave Front Aberration Measurement of the Eye
In contrast with man-made optical systems, human and animal eyes are optical systems in which the individual internal components of a given eye are not normally separately accessible for either direct measurement or adjustment, the output of the optical system is not directly accessible for analysis and the characteristics of individual components change over time with growth, aging and other factors.
The most common reason for measuring the optical characteristics of a human eye is to determine a prescription for corrective lenses to correct vision problems. Such measurements of the optical characteristics of the eye have long been made by the actual or apparent substitution of lenses with various correction factors with the patient indicating the effect of each substitution in terms of the image being clearer or fuzzier. This technique determines an overall correction for the optical characteristics of the eye.
Such determinations are subject to experimental errors and such events as accommodation of the eye to the substituted lens in a manner which gives the impression that a particular correction is desirable, when in fact that correction is not optimum.
Further, these measurement techniques determine corrections which improve overall vision, but are limited in normal practice to prism, cylindrical and spherical corrections which are low order corrections to the patient's actual, detailed vision errors which include higher order terms or characteristics which these measurement techniques cannot determine.
For the most part these prior art measurement systems are subjective and require the active participation of the patient for their success. In such cases the ophthalmologist must rely on the patient to indicate accurately which images are clearer than others as an indication of the appropriate degree of correction. This requirement for active participation of the patient is a disadvantage in a number of circumstances such as in the diagnosis of small children who have difficulty in understanding what is being asked of them and prevents its use for infants who are incapable of indicating the effect of such lens substitutions.
The requirement for the active participation of the patient in the determination of the characteristics of the eye can have unfortunate effects. Some anomalous conditions result in permanent disabilities because they are not detected during infancy because of the inability of infants to communicate with ophthalmologists. For example, if one eye is in focus and the other is severely out of focus during the time the brain is developing its ability to interpret visual signals, then a permanent disability develops in which the out-of-focus eye is never able to contribute usefully to the brain's image recognition because of a lack of proper stimulation during the period in which the brain's image interpretation functions became established. A person suffering from this condition can tell with the affected eye whether the lines in an image are sharp or fuzzy, but cannot assimilate the perceived information into an image. Present subjective refraction measurement systems are incapable of determining the development of this condition in infants because they cannot accurately diagnose the visual acuity of the eye without the active participation of the patient.
A number of objective refractometers have been developed in the hope of overcoming these problems. However, each of these has had problems or deficiencies of its own. One common deficiency is accommodation by the eye being measured. Another common problem is determining and maintaining accurate alignment of the measurement system during the measurement cycle, since any misalignment can cause inaccurate results.
In recent years, substantial interest has developed in using laser sculpturing, i.e. ultraviolet (UV) light laser ablation, to shape the anterior surface of the cornea as a means of providing corrected vision in place of the use of glasses or contact lenses. U.S. Pat. No. 4,665,913, which is incorporated herein by reference, discloses a UV laser scanning ablation technique for shaping a cornea in which a laser beam which produces a small spot is scanned across the cornea to remove a desired thickness of corneal material on each scan. The area scanned is increased or decreased on subsequent passes to scan each portion of the corneal surface a number of times which is proportional to the thickness of material to be removed at that portion of the cornea.
An alternative to the direct shaping of the corneal surface, is to essentially permanently attach a lenticule to the cornea with the lenticule being shaped to provide the desired vision correction. We say "essentially permanent" because the intention is to leave the lenticule in place permanently, unless some problem should develop which requires its removal. Such lenticules themselves may be reshaped or re-profiled by laser ablation at the time of installation or subsequent thereto to compensate for changes in the overall characteristics of the eye. Such techniques are disclosed in more detail in U.S. Pat. No. 4,923,467, entitled, "Apparatus and Process for Application and Adjustable Re-profiling of Synthetic Lenticules for Vision Correction" by Keith P. Thompson, which is incorporated herein by reference.
In addition to the advantages provided by eliminating the need for eye glasses or contact lenses, both of these techniques are conceptually capable of providing substantial additional advantages in that each should, under proper control and with sufficiently detailed correction instructions, be able to produce fully asymmetric reshaping of the cornea in a practical manner, rather than being limited to the sphere, cylinder and wedge approximation mentioned previously. If the spatially resolved refraction data indicated by the scanning lasers is not available, the most effective plan may be to measure the preoperative spherical aberration of the corneal front surface and maintain this same aberration during the laser sculpting of the cornea (see Schwiegerling 1998). If the spatially resolved refraction data were available, the method of Klein (Klein 1998) could be used to plan the optimal scanning pattern to sculpt the cornea.
However, in order to provide such detailed correction, there is a need for measurement techniques which measure the shape of the cornea and the existing refraction characteristics of the eye with the same detail and precision as can be provided by the correction modality in order that the errors may be fully corrected in this manner.
A measurement system providing such a correction measurement should be fast and should measure the eye's detailed refraction characteristics referenced to the cornea as a function of position across the dilated pupil. This position-dependent measurement may be categorized as a spatially resolved refraction measurement because the refraction at each measurement region (point) is determined in that local measurement region independent of the refraction at other, non-overlapping measurement regions.
In Penney (U.S. Pat. No. 5,258,791) and He (He 1998) a flying spot spatially resolved objective autorefractometer is described which solves a number of these issues, but has inherent limitations of its own. For example, the design requires a sequence of measurements be made as a flying spot is scanned. The resulting system is complex and is subject to errors due to patient movement. A similar flying spot system could be constructed using the manual psychophysical system described in (Salmon 1998) which employs Smirnov's principal, which would have the same drawbacks as the Penney system.
Recently, Hartmann-Shack lenslett arrays (also known as micro-lens arrays) have been used to measure the entire wave front aberration of the eye (Salmon 1998, Liang 1997, Liang 1994). This method has much promise for solving the problems associated with the previous techniques. The basic idea for this method of wave front measurement is illustrated in FIG. 15. In FIG. 15 we illustrate an input laser source viewed by the eye such that it forms a diffuse point source at the fovia. This diffuse spot then acts as a point source as it exits the eye. As this wave front passes through the lenslett array, it is focused on the CCD. If the eye had perfect optics, the wave front exiting the eye would form a plane wave. In this case the fovial point source would appear on the CCD as a regular array of points of light and would match perfectly with the reference spot locations obtained during a calibration operation. In FIG. 16 we illustrate the effects of an aberrated wave front. Here the wave front is no longer a plane wave. As the wave front passes through each lenslett, the focused point on the CCD is deviated from the reference position according to the slope of the wave front at the lenslett. This deviation, dy, along with the focal length of the lenslett allows us to compute the local partial derivative of the wave front. The wave front is reconstructed by integrating all derivatives computed for each lenslett in the CCD image. Details of this reconstruction process are provided in the following paragraphs.
Implementations of these lenslett array systems have been limited to research laboratories for several reasons. First, the laser spot on the fovia must be in rather sharp focus on the CCD array to allow reliable measurements. In these laboratory systems, this is usually accomplished by imaging through the subject's spectacle correction. In a clinical setting where exam time is important, finding which spectacle correction to use for each subject may be time prohibitive. A more serious drawback is the problem of point "cross-over" shown in FIG. 17. This occurs when the wave front has so much aberration that the fovia point sources associated with a given lens is mistakenly assigned to a neighboring lenslett. When this happens, the sign and/or magnitude of the partial derivatives of the wave front will contain a huge error. Another issue is the choice of reference axis of the wave front aberration. It was demonstrated by Cui (Chi 1998) that different wave front aberrations result as various reference axes are chosen. Since clinicians will want to register the wave front aberration data with the corneal surface and thickness data, this issue must be resolved for the situation where one instrument is used for corneal measurements and another is used for aberration measurement.
In the present invention, we solve each of the current drawbacks to typical Hartmann-Shack lenslett array based aberration measurement. First, we provide a simple focus adjustment mechanism which can account for at least +/-10 diopters of focus error. We also provide both high-resolution and low-resolution wave front analysis paths, so we can easily and effectively solve the point "cross-over" problem and increase the total number of data points being processed per exam. The complex issue of registering the corneal surface measurements with the aberration measurement is automatically solved by integrating both systems and providing simultaneous measurements in a single exam.
In addition to these individual benefits over the existing state of the art in measuring the corneal front surface, corneal back surface and thickness, and wave front aberration, we integrate these usually separate ocular measurement functions into a single instrument. This provides the additional benefits of:
(1) A more economical system compared to the combined cost of individual instrument.
(2) Reduced exam time since several measurements are made simultaneously.
(3) Data and exams are integrated into the same computer and database.